1. Field of the Invention
The present invention relates to a process for the production of polyarylene sulfides and, more particularly, to a process for the production of polyarylene sulfides in high yields and with high efficiency, which have high molecular weights and contain salt residues in less amounts.
2. Description of Related Art
Polyarylene sulfides such as polyphenylene sulfide and the like are thermoplastic resins in which a portion of their molecules is to be thermoset and which possess excellent characteristics as engineering plastics such as chemical resistance as well as mechanical properties and thermally resistant rigidity in a wide range of temperatures.
U.S. Pat. No. 3,354,129 and Japanese Patent Publication No. 3,368/1970 disclose processes for preparing polyarylene sulfides such as polyphenylene sulfide from an alkali metal sulfide and a polyhalide of an aromatic compound. The processes, however, suffer disadvantages that a yield in the polyarylene sulfides is decreased and a solvent used is lost on account of decomposition of the alkali metal sulfide upon removal of hydrated water of the alkali metal sulfide.
Accordingly, in order to solve these disadvantages, there have been proposed improved methods using an alkali metal hydrosulfide which is readily handled during dehydration operation.
For example, U.S. Pat. No. 3,876,591 discloses a process in which an alkali metal hydrosulfide is used at a rate from 0.8 to 1.5 times as much as a polyhalogen aromatic compound. Examples shown in this patent publication, however, provide polyarylene sulfides which are small in molecular weight and have a maximum inherent viscosity (.eta..sub.inh) of 0.04 at the highest and a melting point of 260.degree. C. or lower. Furthermore, U.S. Pat. No. 3,889,433 discloses a process in which an alkaline earth metal hydroxide or an alkali metal carbonate is added. Examples of this patent publication give only polyarylene sulfides with low molecular weights, which have a inherent viscosity (.eta..sub.inh) of 0.06 or lower and a melting point of 263.degree. C. or lower. It is to be noted that these polyarylene sulfides are said to be inappropriate as engineering materials.
As an improvement in the processes as have been described hereinabove, Japanese Patent Publication No. 14,698/1982 proposes a process in which an alkali metal hydrosulfide and an alkali metal hydroxide are used in particular conditions. This process provides only polyarylene sulfides with a viscosity as low as about 80 to 83 poises, however, such low viscosity polyarylene sulfides are not suited for uses such as films, fibers and the like. Furthermore, although this patent publication fails to state in an expressed manner, the resulting reaction products are subjected to conventional post-processing so that it is presumed that an ash remains in the reaction products in a content as high as approximately 500 to 6,000 ppm as disclosed in U.S. Pat. Nos. 4,025,496 and 4,373,091. For instance, U.S. Pat. No. 4,025,496 discloses a process in which the order of adding an alkali metal hydroxide is specified to provide polyarylene sulfides which can be readily separated. The polyarylene sulfides produced by this process contain an ash in a content as high as 5,600 ppm. The polyarylene sulfides as having such a high ash content are inappropriate for uses as molding materials, films, filaments and, particulary, electronic parts. Furthermore, this process requires an additional dehydration operation because an aqueous solution of an alkali metal hydroxide is added after dehydration of an alkali metal hydrosulfide so that dehydration operation should be carried out in a two-step method. This is extremely laborious. It is further to be noted that this process should be improved because the alkali metal hydrosulfide is caused to decompose to a considerable extent.
As a process for removing a salt residue from polyarylene sulfide products, U.S. Pat. No. 4,071,509 proposes a process for recovering polyarylene sulfides with a low ash content, in which the polyarylene sulfide products are washed with an organic amide compound in the presence of an alkali metal salt. This process involves contacting the alkali metal salt with the resulting polyarylene sulfide after completion of polymerization, however, this process requires a complex purification operation.
In order to improve this process, U.S. Pat. No. 4,810,773 proposes a process in which an alkali metal halide is used in place of the alkali metal salt. It is to be noted, however, that effects are insufficient.
Furthermore, U.S. Pat. No. 4,038,263 discloses a process using a lithium halide as a catalyst for the reaction of an alkali metal sulfide with p-dichlorobenzene. This process can provide polyarylene sulfides with a high molecular weight yet in a yield as low as from about 70% to 85%. Thus demands have been made to further improvements in yields. This process also suffers the disadvantages that dehydration operation is difficult because the alkali metal sulfide is used and that handling is also difficult because the alkali metal sulfide is used in the form of a solid substance.